1. Field of the Invention
This invention relates generally to a multilayer film and, more specifically, to an easily producible, heat-shrinkable polyolefin multilayer film having excellent heat-shrinkability and tearing strength.
2. Background of Invention
In order for a heat-shrinkable film to easily provide a tight fit package, the film is required to have both high heat-shrinkability and high mechanical strengths, such as a tearing strength and resistance to breakage during heat sealing and during use. In addition, the film with such desired properties must be produced without difficulty.
A heat-shrinkable film in a tubular form is generally produced by a process in which a thermoplastic resin melt is extruded through a die to form a tubular preform, the preform being rapidly cooled with water, then reheated to a temperature lower than the melting point thereof but higher than the softening point or glass transition point thereof and stretched by a blown bubble method. The resulting biaxially oriented film when heated shows a property to return to the original dimensions.
The characteristics of the heat-shrinkable film, therefore, depend not only upon the physical properties of the raw material thermoplastic resins but also upon stretching conditions such as the stretching temperature and the drawing ratio. For example, a heat-shrinkable film formed mainly of a polypropylene having a high melting point, a high degree of crystallinity and good stretching processability is not good in low temperature shrinkability and has a low maximum shrinkage value, although the tensile strength and shrinkage stress of the film are high. In contrast, a heat-shrinkable film formed mainly of a resin having a low melting point and low degree of crystallinity, such as a low density polyethylene (hereinafter referred to as LDPE), a linear low density polyethylene (hereinafter referred to as LLDPE), a very low density linear polyethylene (hereinafter referred to as VLDPE) or an ethylene/vinyl acetate copolymer (hereinafter referred to as EVA), exhibits low temperature shrinkability, a low shrinkage stress and excellent heat sealability. However, since such a low melting point resin is stretchable only in a narrow temperature range, it is necessary to strictly control the stretching temperature or to cross-link the resin by irradiation with an electron beam.
To cope with the foregoing problems, there have been proposed various kinds of heat-shrinkable films in which two or more different resins are used in combination as a blend and/or as a laminate.
U.S. Pat. Nos. 4,194,039 and 4,229,241 disclose that a heat-shrinkable, three layer film composed of two skin layers of an ethylene/propylene copolymer and a core layer of a blend of an ethylene/vinyl acetate copolymer with an ethylene/butylene copolymer shows such a shrinkage stress, clarity, sealing temperature, a wide temperature range for shrinking and a tearing strength that would not be obtainable with a polyolefin single layer film.
U.S. Pat. No. 4,532,189 discloses a heat-shrinkable multilayer film having a core layer of LLDPE or a linear medium density polyethylene (hereinafter referred to as LMDPE) and showing improved mechanical strengths.
U.S. Pat. No. 4,551,380 discloses a heat-shrinkable three layer film having a core layer of a blend of LLDPE, LMDPE and EVA and exhibiting good heat sealability and shrinkability.
U.S. Pat. No. 4,820,557 discloses a high abuse-resistant, heat-shrinkable film including at least one layer of an ethylene/alpha-olefin copolymer having a density of 0.935 g/cm.sup.3 or less and a lower I.sub.10 /I.sub.2 ratio in comparison with the corresponding ethylene/alpha-olefin which has the same comonomer, the same density and the same melt index at 190.degree. C. and 2.16 kg loading.
U.S. Pat. No. 5,059,481 discloses a puncture-resistant, heat-shrinkable film which includes VLDPE having a density of 0.86-0.91 g/cm.sup.3 and a 1% modulus of below 140,000 kPa and which shows a shrinkage value of 30-50%.
U.S. Pat. No. 4,837,084 discloses that a heat-shrinkable multilayer film containing a layer of a copolymer of ethylene and an alpha-olefin with six or more carbon atoms having a density of 0.910 g/cm.sup.3 or less and a melt index of 2 or less exhibits excellent heat-shrinkability, orientation speed properties and abuse-resistant properties.
U.S. Pat. No. 4,302,557 discloses a cold drawn film having excellent low temperature shrinkability and formed of a blend of (A)+(B), (B)+(C) or (A)+(B)+(C), wherein (A) is a non-rigid polyolefin resin such as EVA, (B) is an ethylene/alpha-olefin elastomer and (C) is a rigid polyolefin resin such as polypropylene.
U.S. Pat. No. 5,272,016 discloses that a heat-shrinkable multilayer film having opposite outer layers each formed of a blend of VLDPE (density of 0.914 g/cm.sup.3 or less) with an ethylene/alpha-olefin plastomer (density of 0.90 g/cm.sup.3 or less) is suited for overwrapping trays.
U.S. Pat. Nos. 4,923,722, No. 5,001,016 and 4,948,657 disclose that a heat-shrinkable multi-layer film having a core or inner layer of a blend of polypropylene with VLDPE and outer layers each formed of polypropylene shows good low temperature shrinkability. The tearing strength of such a layer is 4.0-11.6 kg/cm (tearing load is 8.0-23.2 g at 20 .mu.m) which is lower than that of a heat-shrinkable film formed mainly of LLDPE, VLDPE or EVA.